Re: [Haskell-cafe] Why can't we make an instance declaration on a type synonym?

On Sun, Jan 3, 2010 at 2:22 AM, Ivan Lazar Miljenovic wrote:

jberryman writes:

...

This may be a dumb question, but why can we not declare a Monad instance of a type synonym? This question came to me while working with the State monad recently and feeling that the requirement that we wrap our functions in the State constructor is a bit... kludgy.

Because type defines an _alias_.  If you define "type Foo = Maybe Int", then everywhere you have a "Foo" the compiler should be able to replace it with "Maybe Int".

As such, if you have a custom instance on your type synonym (say a custom Show instance for Foo), then which instance will the compiler use?

Thanks. I guess what I'm really asking is if there is any way to redefine the monad instance for (->) such that we can have a State monad without the data constructor wrapper. It sounds like there probably isn't, but it seems like that would be a pretty useful thing to be able to do generally.